Volcanic lake systems as terrestrial analogue for sulphate-rich terrains on Mars

Abstract

Remote-sensing observations and rover missions have documented the abundant presence of sulphate-rich mineral associations on the surface of Mars. Together with widespread occurrences of silica and frequent enrichments of chlorine and bromine in soils and rocks, the sulphate associations are fingerprints of acidic fluids involved in interaction processes with rocks at or near the planet’s surface. This thesis addresses the question if active volcanic-hydrothermal systems on Earth are plausible analogues of geological settings where sulphates formed in the ancient history of Mars. Field evidence and geochemical modeling have been used to demonstrate how such acid volcanic fluids can produce mineral assemblages with a range of Ca-, Al-, Fe- and Mg-sulphates through temperature changes, interactions with rocks and minerals or evaporation. Mars-like secondary mineral associations were discovered in a wide variety of sub-environments. At Copahue volcano (Argentina), they are distributed along a ca. 40 km long stretch of an acidic stream that has its source in the summit area, passes a glacial lake, and receives input from neutral tributaries. At Poás volcano (Costa Rica), the sub-environments include a hyperacid crater lake, hot springs, fumarole vents and areas exposed to acid rain or spray. Minerals detected in both settingsthat also occur on Mars include mono- and polyhydrated ferrous and magnesium sulphates, jarosite-alunite group minerals, hydrated ferric and aluminium sulphates, hematite and other oxides, schwertmannite, gypsum/anhydrite, silica, sulphides, clay minerals and halides. Alunite is uncommon but is an important mineral in the hydrothermal reservoirs below the surface. The relative proportions of rock and acidic fluid during interaction play an essential role in the formation of the sulphates. High rock/water ratios and the presence of olivine are key factors for the formation of Mg- and Fe-sulphates. Modeling results indicate that evolution of acidic fluid in a cyclical sequence of interaction with olivine and evaporation in an open system is required to stabilize Mg-rich sulphates. Temporal changes in δ37Cl, Cl-Br concentrations and Br/Cl ratios in the hyperacid lake water and subaerial fumaroles of Poásreflect interactions between magma-derived gas and liquid water in the volcanic-hydrothermal system below the crater, and provide insight into the status of activity of the volcanic-hydrothermal system at depth. Experiments on hyperacid brine water from Kawah Ijen crater lake (Indonesia), in combination with geochemical modeling, demonstrate that rising chloride and sulphate concentrations promote the formation of complexes with cations, which inhibits the saturation of major solid phases with exception of silica, gypsum/anhydrite, anatase and hydrogen chloride. The effects of HCl evaporation and associated chlorine isotopic fractionation were studied by monitoring temporal changes in the composition of the residual liquid. A marked change in sign and magnitude of fractionation between dissolved and gaseous chloride (HCl) in the course of the experiment was detected, which can be explained by a dependence of the fractionation mechanism on the species distribution of dissolved chloride as a function of pH.